This invention relates to new and improved anaerobic digestion modules useful as components of a system for production of methane gas by anaerobic digestion of feed materials containing suitable organic materials. In particular, this invention relates to vertically stacked anaerobic digestion modules which comprise an anaerobic digestion sub-system suitable for use with the system disclosed in the closely related U.S. Patent Application Ser. No. 56,545, filed by David J. Fischer, et al. on July 11, 1979, for a System and Process for Anaerobic Digestion, which is hereby incorporated by reference.
The system for production of methane gas by anaerobic digestion disclosed and claimed in Application Ser. No. 56,545 is comprised of a feed material pre-treatment sub-system, a slurry preparation sub-system, an anaerobic digestion sub-system, a gas collection sub-system, a gas storage sub-system, a sludge separation sub-system and a particle separation sub-system. The anaerobic digestion sub-system disclosed in that application is comprised of a plurality of individual digestion modules arranged to accommodate a multi-stage anaerobic digestion process. In the preferred embodiment disclosed in that application, each individual digestion module is a cylindrically shaped reaction vessel, such as a conventional fifty-five gallon drum, positioned on its side with its slurry inlet port at a slightly lower elevation than the slurry outlet port of the preceding digestion module. In addition, each individual digestion module is positioned on an incline with its slurry outlet port slightly lower than its slurry inlet port.
The structural arrangement of digestion modules disclosed in Application Ser. No. 56,545 allows gravity transfer of the digesting slurry from one digestion module to the next successive digestion module. It can be readily appreciated that such a structural arrangement of digestion modules does not require pumps for transferring the digesting slurry. Also, with such a structural arrangement, no mechanical mixers are necessary for mixing the digesting slurry in each digestion module. Sufficient mixing occurs when the digesting slurry is transferred from one digestion module to the next successive digestion module.
While an anaerobic digestion sub-system utilizing the structural arrangement of digestion modules disclosed in the Application Ser. No. 56,545 has these and other advantages, an extensive structural frame is required for its vertical support. And, it is desirable to include an operator walkway as a component of the structural frame. Since the number of individual digestion modules in the anaerobic digestion sub-system disclosed in that application varies depending upon the particular feed material to be digested, a standard design for the structural frame for those modules is not possible. Furthermore, a structural frame of the type which is required to support those modules does not lend itself to fabrication in a manufacturing plant prior to field installation of the modules. For these reasons, expensive custom design and field fabrication will be required for most installations of anaerobic digestion sub-systems using the structural arrangement of digestion modules disclosed in Application Ser. No. 56,545.
While an anaerobic digestion sub-system using the structural arrangement of digestion modules disclosed in Application Ser. No. 56,545 does not have to be installed inside a building, it is better practice to do so. First, it is difficult to maintain and control the temperature in the digestion modules if the sub-system is installed outside. Next, such a structural arrangement of digestion modules exposes a substantial surface area to the elements. Unless either expensive stainless steel construction or inside installation is used, frequent cleaning and maintenance of the modules and their structural frame will be required.
It will be readily appreciated that the structural arrangement of digestion modules disclosed in Application Ser. No. 56,545 requires extensive piping for interconnection of the individual modules. Not only are expensive piping materials required, but also expensive field welding operations are required to fabricate the anaerobic digestion sub-system. In many countries where anaerobic digestion systems would be desired, it is often difficult to employ qualified field construction welders.
It is desirable to have a plurality of anaerobic digestion modules which comprise an anaerobic digestion sub-system having all of the advantages of the structural arrangement of digestion modules disclosed in Application Ser. No. 56,545 without the foregoing and other disadvantages inherent in such a structural arrangement. Preferably, the operating characteristics of the resulting sub-system would be identical or superior to those of the sub-system disclosed in that application. It is also desirable to have a modular anaerobic digestion sub-system capable of being fabricated in a manufacturing plant and transported by barge, boat, rail or truck shipment to the site selected for installation of the anaerobic digestion system using the sub-system. And, of course, it is desirable for such a sub-system to be comprised of standard components which enable the system designer to easily vary the number of digestion modules in the sub-system without major perturbations in the normal operations of the manufacturing plant which fabricates such sub-systems.